如何在Elmish.WPF/Elmish中从自定义事件向调度器发送消息？

问题场景 #

你定义了一个单例事件中介类：

type EventMediator private () =
    let mutable _name = "";

    static member val private _instance = lazy EventMediator()
    static member Instance = EventMediator._instance.Value

    member val private nameChanged = Event<unit>()
    member this.NameChanged = this.nameChanged.Publish

    member this.Name
        with get() = _name
        and set(value) =
            _name <- value
            this.nameChanged.Trigger()

尝试在外部模块中订阅事件并发送Elmish消息时，因回调处于Update循环外，直接调用dispatch无效：

let p = EventMediator.Instance
p.NameChanged.Add(fun () -> dispatch RequestAppointmentsMsg )  // 无效

相关定义：

type Msg =    
    | RequestAppointmentsMsg 

Update循环签名：

update (msg:Msg) (m:Registration) : Registration * Cmd<Msg> = ....

解决方案 #

方法1：将Dispatch函数注入到事件中介 #

修改单例类，添加存储dispatch函数的字段与设置方法，在Elmish程序启动时传入有效dispatch：

1. 更新EventMediator：

type EventMediator private () =
    let mutable _name = ""
    let mutable _dispatch : (Msg -> unit) option = None

    static member val private _instance = lazy EventMediator()
    static member Instance = EventMediator._instance.Value

    member val private nameChanged = Event<unit>()
    member this.NameChanged = this.nameChanged.Publish

    // 添加设置dispatch的方法
    member this.SetDispatch(dispatch) =
        _dispatch <- Some dispatch

    member this.Name
        with get() = _name
        and set(value) =
            _name <- value
            this.nameChanged.Trigger()
            // 可直接在这里触发消息，无需外部订阅
            match _dispatch with
            | Some d -> d RequestAppointmentsMsg
            | None -> ()

2. 在Elmish程序初始化时传入dispatch：

Program.mkProgram init update view
|> Program.withWpfConfig config
|> Program.runWith (fun dispatch ->
    EventMediator.Instance.SetDispatch dispatch
)

若仍需外部订阅事件，可直接使用存储的dispatch：

let p = EventMediator.Instance
p.NameChanged.Add(fun () ->
    match p.Dispatch with
    | Some d -> d RequestAppointmentsMsg
    | None -> ()
)

方法2：用Elmish的Cmd订阅外部事件 #

利用Elmish的Cmd.ofSub，在初始化或消息处理逻辑中订阅外部事件，确保回调处于Elmish上下文内，可直接发送消息：

1. 在init函数中添加订阅逻辑：

let init () : Registration * Cmd<Msg> =
    let initialModel = { ... }
    // 封装订阅事件的Cmd
    let subscribeEvent = Cmd.ofSub (fun dispatch ->
        let p = EventMediator.Instance
        p.NameChanged.Add(fun () -> dispatch RequestAppointmentsMsg)
    )
    initialModel, subscribeEvent

这种方式更贴合Elmish架构设计，所有外部事件订阅通过Cmd统一管理，避免全局状态依赖。

内容的提问来源于stack exchange，提问作者Alan Wayne